A “time cloak” technique that makes an event undetectable (albeit on the picosecond scale) is a step toward the development of spatio-temporal cloaking, engineers at Cornell University report. Applied engineering and physics professor Alexander Gaeta and colleagues found that the trick is to create a gap in a beam of light, to have the hidden event occur as the gap goes by and then to stitch the beam back together. It all happens way too quickly to be seen by the human eye: in this case 15 ps, or 15 trillionths of a second (1 ps is to 1 s as 1 s is to 31,700 years). The optical fiber-based system steers light “around” an event so that no evidence (such as a change in the temporal or spectral properties of the light beam) is detectable. They used a split-time lens to break light into its slower (red) and faster (blue) components, creating a tiny temporal gap by speeding up and slowing down different parts of the beam. The gap created might be increased up to 10 ns, they say, and the technique could have applications in fiber optic data transmission, such as inserting an emergency signal without interrupting the main data stream, and in data processing, such as multitasking operations in light-based computers. This video simulates the time cloaking device developed by Cornell University researchers. A ball is trying to pass through a green beam of laser light without being detected. Two short pulses of red laser light change the color of the green light, and because different colors travel at different speeds, a gap is opened in the beam exactly when the ball is passing through. Then the opposite manipulation closes the gap, and two other pulses change the light back to green. Hence, the ball manages to go through the beam without being detected. (Video: Gaeta lab/Cornell University) The experiment was inspired by a theoretical proposal published by Martin McCall, professor of physics at Imperial College London, in the Journal of Optics in November 2010 (See: ‘Space-time’ Cloak Could Hide History). “But his method required an optical response from a material that does not exist,” Gaeta said. “Now we’ve done it in one spatial dimension. Extending it to two [hiding a moment in an entire scene] is not out of the realm of possibility. All advances have to start from somewhere.” The research, funded by DARPA and the Cornell Center for Nanoscale Systems, appears in the Jan. 5 issue of Nature. For more information, visit: www.cornell.edu